1. Field of the Invention
The invention disclosed herein relates to acoustic measurements and, in particular, to performing the measurements in a borehole.
2. Description of the Related Art
Exploration and production of hydrocarbons generally requires accurate and precise measurements of earth formations, which may contain reservoirs of the hydrocarbons. The reservoirs are accessed by drilling boreholes into the earth formations. Well logging is one technique used to perform the measurements from within the boreholes.
In one type of well logging referred to as wireline logging, a logging tool supported by an armored cable is conveyed through a borehole. The armored cable generally contains electrical cables for supplying power to the logging tool and communicating with the tool. The logging tool includes those components such as sensors and processors used to perform the measurements. As the logging tool is conveyed through the borehole, the measurements are performed at various depths. The measurements are associated with the depths at which they were performed and displayed as a log.
Various types of measurements can be made to produce a log. One type of measurement involves measuring the velocity of sound in an earth formation. Many characteristics of the earth formation such as type of a material, amount of a material, and porosity of a material can be estimated by knowing the velocity of sound in the earth formation as a function of depth.
For example, a sound wave may be emitted that penetrates the earth formation and is reflected back. If it is known that the earth formation is composed of a certain type of material and that the pore spaces of the material are filled with water, then it is possible to determine the porosity based on a measurement of the speed of the sound wave.
An acoustic logging tool is used to measure the velocity of sound downhole. In general, the acoustic logging tool includes at least one acoustic transmitter to emit a sound wave, at least one acoustic receiver to receive the sound wave, and a processor to process data from the tool to estimate the velocity of the sound wave. The transmitter and receiver may each be referred to as an acoustic transducer. A conventional acoustic transducer for use downhole operates over a range of about ten to fourteen kilohertz. Unfortunately, a wider range especially on the low side is more desirable for the many types of acoustic measurements that can be performed downhole.
Therefore, what are needed are techniques for transmitting and receiving a sound wave having a wide frequency range downhole. Preferably, the sound wave can cover the frequency range of one to fifteen kilohertz.